Siliconizing of electrical sheet steel by diffusion



United States Patent SILICONIZING OF ELECTRICAL SHEET STEEL BY DIFFUSKONKlaus Sixtus, Bad Hamburg, and Helmut Lammel,

Frankfurt am Main, Germany, assignors to LicentiaPatent-Verwaltungs-G.m.b.H., Frankfurt am Main, Germany, No Drawing.Filed Nov. 21, 1962, Ser. No. 239,340 Claims priority, applicationGermany, Nov. 29, 1961,

15 Claims. (Cl. 14$113) The present invention relates to a method oftreating sheet metal for the purpose of improving its magneticcharacteristics, particularly iron sheets used as laminations inelectrical machinery and containing, in addition to the iron, 4.5% oreven only up to 3.5% silicon, depending on whether the sheets are to beonly hot-rolled or whether, after the hot-rolling, they are still to becoldrolled.

It is generally desirable to improve the good magnetic characteristicsof sheet iron, which are obtained in the case of cold-rolled sheets andwhich are related with the formation of special crystal textures, byincreasing the silicon content above the customary level. It is knownthat with increasing silicon contents, firstly, the specific electricresistance increases, as a result of which that portion of AC. fieldlosses which is due to eddy currents is decreased, and secondly, themagnetostriction is reduced, i.e., the changes of length of theferromagnetic substance in the magnetic field become smaller, it beingthese changes in length, for instance, which cause the undesired noiseproduced by transformer laminations. If, however, the sheets are to becold-rolled, the starting material cannot contain more than about 3.5%silicon, because a silicon content above this percentage renders thematerial too brittle to allow it to be cold-rolled.

It is, therefore, an object of the present invention to provide a methodby means of which the silicon content can be increased above thecustomary limit despite the fact that such a material cannot, for allpractical purposes, be cold-rolled.

With the above object in view, the present invention resides in a methodof improving the magnetic characteristics of sheets for use inelectrical apparatus, wherein sheets of customary silicon content whichhave been put through the cold-rolling process are thereafteradditionally siliconized by annealing in a SiCh-containing non-oxydizingprotective gas, after which the sheets are subjected to a homogenizingtreatment carried out by means of a second annealing step.

While it has been suggested to react the sheets with silicon so as toprovide the sheets with a protective (Fe Si) layer containing about 14%silicon, the sheets as a whole are not, in this way, homogeneouslysiliconized. Even if the surface of the sheets, which have been enrichedto a silicon content of 14%, are then homogenzied by annealing at hightemperature, the sheets will not have the desired good magneticcharacteristics.

The method according to the present invention is able to raise thesilicon content of the sheets to approximately 7%, this silicon contentbeing distributed evenly over the entire cross section of the sheets. Ithas been found that this is possible by annealing, i.e., heat treatingthe sheet metal in a SiCl -containing protective gas atmosphere "icewhich has but a relatively small proportion of silicon tetrachloride,preferably only up to 4% by volume. The protective carrier gas may, forinstance, be nitrogen or argon. The same should be sutficiently pure sothat no oxide layer is formed at the surface of the sheets, because suchan oxide layer would impede a homogeneous siliconization. For thisreason, the sheets to be siliconized should first have any oxide layersremoved. During the siliconization, the sheets are preferably kept at atemperature of between 1100 and 1250 C. Expressed in other words, thesilicon is diffused into the sheet metal and, so long as the proportionof silicon tetrachloride is kept no greater than 4% by volume and thetemperature at which the siliconization--this term being used throughoutthe instant specification to refer to a diffusion rather than a platingprocesstakes place is kept within the above-mentioned limits, the rateat which silicon is deposited is less than the rate at which ironsilicide (Fe Si) is formed, so that there is no opportunity for any ironsilicide layer to be formed in the first place, instead, the siliconwill diffuse into the metal. It is in the course of this annealing step,then, that the silicon is deposited and diff-used into the sheet metal.

Even though the above-described process will produce what is for allpractical purposes an even siliconization throughout the entire crosssection of the sheets, it has been found expedient to subject thesiliconized sheets to a homogenizing process by annealing attemperatures of between 1150 and 1300 C.

Finally, the magnetic characteristics can be improved even further byheating the homogenized sheets to a temperature of about 800 C. andthereafter cooling them in a magnetic field. The magnetic field shouldhave a field strength of more than 10 oersteds and be applied in such amanner as to be oriented in the preferred magnetic direction as dictatedby the desired crystalline structure, i.e., the lines of fiux or" themagnetic field should be so oriented as to produce the desiredcrystalline and hence magnetic anisotropy of the sheets. The sheets canbe cooled at a rate of for example C. per minute.

The siliconization can be carried out in a furnace which surrounds agas-tight tube. The strip of sheet metal to be siliconized is passedthrough the tube while there flows through this tube a gas mixturecomposed of the silicon tetrachloride and the protective carrier gas,e.g., nitrogen or argon. The amount of SiCl, which is drawn out of thesilicon tetrachloride reservoir, the latter being kept at a constanttemperature, depends on the temperature and speed of the gas, and can beadjusted to any desired amount by changing these variables. tice, itwill be most expedient to keep the proportion of the silicontetrachloride under 4% by volume.

The following are several examples of the present invention. Thestarting material was constituted by sheets usable for transformerlaminations, which sheets, after being cast, were cold-rolled to athickness of 0.32 millimeter. The silicon content was 3.2%. Previouslyformed oxide layers were removed by etching in diluted hydrochloric acidwith bromide admixed.

During the siliconization, the furnace was moved at a constant speedalong the sheet web. The annealing zone was 5 centimeters long, thetemperature was approximately 1150 C.

In prac- Percent by Speed of the Silicon content, volume of SiCL;furnace, centipercent by in N 2 meters per hour weight The siliconizedsheets were then rendered homogeneous by annealing for hours at 1200 C.in a hydrogen atmosphere, and thereafter cooled to room temperature. Thesheets were then again heated to 800 C., from which temperature theywere cooled to room temperature at a rate of approximately 100 C. perhour, this cooling being done in a magnetic field having a strength ofapproximately 200 oersteds.

By means of a strip measuring apparatus, the following values for themagnetic reversal losses at 10 kg. (V at 50 c.p.s. were measured in thecase of non-siliconized and siliconized sheets, both with and withoutcooling in the magnetic field.

Silicon content, Without cooling After cooling percent by in magneticfield in magentie field weight V10, W/kg. Vno, W/kg.

It will be seen from the above that the V -value is decreased byincreasing the silicon content and thereafter cooling the sheets in themagnetic field. With the latter step, the silicon contents has to beapproximately 7% before the V -value is reduced.

The minimum magnetostriction saturation was found in sheets having asilicon content of approximately 6%.

It will be understood from the above that, thanks to the presentinvention, sheets of relatively high silicon contents are obtainedwithout it being necessary, however, to forego the cold-rolling processfor making the sheets. This is accomplished, as described above, byfirst completing the cold-rolling process of the sheet metal having asilicon content sufficiently low to allow the sheet metal to have beencold-rolled in the first place, i.e., about 3.5%, and only thereaftersubjecting the thus cold-rolled sheets to a treatment which raises thesilicon contents to the desired level, the latter being a level abovethat permissible for cold-rolling.

The present invention is not limited specifically to the use of silicontetrachloride as the voltatile silicon compound, the latter term beingdeemed to refer to a silicon compound which is volatile at the annealingtemperature during which sheet metal is siliconized. For example, thevolatile silicon compound can be one of the other silicon halides,namely, SiFl SiBr and 5H,, or silicon tetraacetate, silicon sulfidedibromide (SiSBr or silane (SiH or one of the substituted silanes, e.g.SiCl H or silicon tetrathiocyanate (Si(SCN) As stated above, if thesilicon compound is silicon tetrachloride, the proportion of suchsilicon tetrachloride will be no greater than 4% by volume; this, itwill be of 4%, or it being this percentage, rather than the percentageof SiCl, as such, which is of primary significance insofar as volatilesilicon compounds in general are concerned.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes, andadaptations, and the same are intended to be comprehended within themeaning and range of equivalents of the appended claims.

For instance, the siliconized sheets need not be cooled to roomtemperature after being rendered homogeneous by annealing, but may becooled to approximately 800 C., from which temperature they are cooledto room temperature in a magnetic field as described above.

What is claimed is:

1. A method of improving the magnetic characteristics of cold-rolledsheet metal having a silicon content sufficiently low to allow saidsheet metal to have been coldrolled, said method comprising the step ofsubjecting the sheet metal to a temperature of between 1100 and 1250" C.in an atmosphere of a gas mixture composed of a non-oxydizing carriergas and a volatile silicon compound, wherein the total silicon contentsin said gas mixture is no greater than /a% by volume of the total gas,to diffuse silicon into the sheet metal to a level above thatpermissible for cold-rolling; and homogenizing the diffused silicon byannealing the sheet metal.

2. A method as defined in claim 1 wherein said volatile silicon compoundis silicon tetrachloride, the proportion of said silicon tetrachloridein said mixture is no greater than 4% by volume of the total gas.

3. A method as defined in claim 2 wherein said carrier gas is selectedfrom the group consisting of nitrogen and argon.

4. A method as defined in claim 2 wherein said carrier gas is ofsutficient purity to avoid the formation of an oxide layer on the sheetmetal.

5. A method as defined in claim 2 wherein the annealing during saidhomogenizing step is carried out at a temperature of between 1150 and1300 C.

6. A method as defined in claim 5 wherein said hompgenizing step iscarried out while the sheet metal is in an atmosphere of hydrogen.

7. A. method as defined in claim 2, comprising the further steps ofheating the sheet metal subsequent to the homogenizing step, andthereafter cooling the sheet metal while the same is exposed to amagnetic field.

8. A method as defined in claim 7 wherein the sheet metal, during thesubsequent heating step, is heated to a temperature of approximately 800C., and wherein during said cooling step the sheet metal is cooled at arate of approximately C. per minute.

9. A method as defined in claim '7 wherein the magnetic field has astrength greater than 10 oersteds.

10. A method as defined in claim 7 wherein said magnetic field has astrength of approximately 200 oersteds.

11. A method of improving the magnetic characteristics of cold-rolledsheet metal having a silicon content of not more than approximately3.5%, said method comprising the steps of:

(a) subjecting the sheet metal to a temperature of between 1100 and 1250C. in an atmosphere of a gas mixture composed of a nonoxydizing carriergas and silicon tetrachloride, the proportion of silicon tetrachloridein said gas mixture being not greater than 4% by volume, to diffusesilicon into the sheet metal to a level above that permissible forcoldrolling; and thereafter (b) homogenizing the dilfused silicon byannealing the sheet metal at a temperature of between 1150 and 1300 C.

12. A method as defined in claim 11, comprising the further steps of:

(c) heating the sheet metal to a temperature of about 800 C.; andthereafter (d) cooling the sheet metal to room temperature at a rate ofapproximately 100 C. per hour while the sheet metal is in a magneticfield having a strength of approximately 200 oersteds.

13. A method of improving the magnetic characteristics of cold-rolledsheet metal having a silicon content of not more than approximately3.5%, said method comprising the steps of:

(a) subjecting the sheet metal to a temperature of between 1100 and 1250C., in an atmosphere of a gas mixture composed of a nonoxydizing carriergas and silicon tetrachloride, said carrier gas being selected from thegroup consisting of nitrogen and argon and being of sufiicient purity toavoid the formation of an oxide layer on the sheet metal, and theproportion of silicon tetrachloride in said gas mixture being notgreater than 4% by volume, to difiuse silicon into the sheet metal to alevel above that permissible for cold-rolling; thereafter (b)homogenizing the diffused silicon by annealing the sheet metal forapproximately ten hours at a temperature of between 1150 and 1300 C.while the sheet metal is in an atmosphere of hydrogen;

(c) after cooling of the sheet metal, re-heating the same to atemperature of approximately 800 C; and thereafter (d) cooling the sheetmetal to room temperature at a rate of approximately 100 C., per hourwhile the sheet metal is in a magnetic field having a strength ofapproximately 200 oersteds.

14. A method of making sheet metal for use in electrical apparatus, saidmethod comprising the steps of:

(a) manufacturing the metal with a silicon content sufficiently low toallow the metal to be cold-rolled;

5 (b) cold-rolling the metal;

(c) subjecting the sheet metal to a temperature of between 1100 and 1250C. in an atmosphere of a gas mixture composed of a non-oxydizing carriergas and a volatile silicon compound, wherein the total silicon contentsin said gas mixture is no greater 10 than /s% by volume of the totalgas, to diffuse silicon into the sheet metal to a level above thatpermissible for cold-rolling; and

(d) homogenizing the ditfused silicon by annealing the sheet metal.

15. The method defined in claim 14 comprising the further steps of:

(e) re-heating the sheet metal; and

(f) cooling the sheet metal at a rate of approximately 100 C. per hourwhile the sheet metal is in a magnetic field having a strength ofapproximately 200 oersteds.

References Cited by the Examiner UNITED STATES PATENTS 1,932,306 10/1933Freeland 148-110 2,109,485 3/1938 Ihrig 117-107 2,165,027 7/1939 Bitter148-111 2,318,011 5/1943 Parsons et al. 148-16 2,351,922 6/1944 Burgwin148-110 2,438,892 4/1948 Becker 117-106 2,453,539 11/1948 Reardon148-110 2,501,051 3/1950 Henderson et al 117-106 2,599,340 6/1952Littmann et al. 148-111 2,897,093 7/1959 Eckman 117-106 DAVID L. RECK,Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,224909 December 21, 1965 Klaus Sixtus et a1.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 3, line 46, for "With" read Without Signed and sealed this 29thday of November 1966.

( AL) Attest:

ERNEST W. SWIDER Attesting Officer Commissioner of Patents EDWARD J.BRENNEIt

1. A METHOD OF IMPROVING THE MAGNETIC CHARACTERISTICS OF COLD-ROLLEDSHEET METAL HAVING A SILICON CONTENT SUFFICIENTLY LOW TO ALLOW SAIDSHEET METAL TO HAVE BEEN COLDROLLED, SAID METHOD COMPRISING THE STEP OFSUBJECTING THE SHEET METAL TO A TEMPERATURE OF BETWEEN 1100* AND 1250*C. IN AN ATMOSPHERE OF A GAS MIXTURE COMPOSED OF A NON-OXYDIZING CARRIERGAS AND A VOLATILE SILICON COMPOUND, WHEREIN THE TOTAL SILICON CONTENTSIN SAID GAS MIXTURE IS NO GREATER THAN 2/3% BY VOLUME OF THE TOTAL GAS,TO DIFFUSE SILICON INTO THE SHEET METAL TO A LEVEL ABOVE THATPERMISSIBLE FOR COLD-ROLLING; AND HOMOGENIZING THE DIFFUSED SILICON BYANNEALING THE SHEET METAL.